What it is
Osteocalcin is a small protein made by osteoblasts, the cells that lay down new bone. Its older name, bone Gla protein, describes what makes it unusual: three of its amino acids are gamma-carboxyglutamic acid (Gla) — glutamate with an extra carboxyl group added after the chain is made.
| UniProt P02818 | |
|---|---|
| Precursor | 100 residues |
| Signal peptide | 1–23 |
| Propeptide | 24–51 |
| Mature osteocalcin | 52–100 → 49 amino acids |
| Gla residues | precursor positions 68 (partial), 72, 75 |
At 49 residues it is just over the 40-amino-acid line above which US biologics rules (21 CFR 600.3) treat a chain as a protein — the peptide-or-protein entry explains the line. The library carries it because readers meet it as a blood test and as the centre of a much-cited hormone theory.
What it does in bone
The addition of the three Gla groups is a vitamin K-dependent step. UniProt's annotation states that once carboxylated, osteocalcin "binds strongly to apatite and calcium" — apatite being the mineral that makes bone hard — and that this modification "is essential for bone metabolism". It describes carboxylated osteocalcin as 1–2% of total bone protein, one of the main organic components of the matrix after collagen, and as a regulator that limits bone formation (the latter from animal work).
The blood test
Some osteocalcin escapes into the blood, and because osteoblasts make it, the level is read as a marker of bone formation. That is the role it plays in clinical research: PubMed indexes 1,169 randomised trials that name osteocalcin in the title or abstract, and this library found none that gave osteocalcin itself as a treatment — it appears as a measured outcome, to see whether something else changed bone turnover.
Reference ranges vary with the laboratory, the assay, age and sex. A result belongs with the clinician who ordered it; nothing on this page is a guide to reading one. Its partner marker for bone breakdown is the C-telopeptide test.
The hormone theory — and the dispute
In 2007 a Columbia University group published in Cell (Lee et al., PMID 17693256) that mice lacking osteocalcin had decreased insulin-producing beta-cell proliferation, glucose intolerance and insulin resistance, and that osteocalcin could improve glucose tolerance in mice. The paper's conclusion was that "the skeleton exerts an endocrine regulation of sugar homeostasis". Work that followed, summarised in UniProt's annotation, assigned the uncarboxylated form further hormone roles: raising testosterone through the GPRC6A receptor on testicular Leydig cells, and acting on the brain through GPR158.
Those findings came largely from one knockout mouse line. In 2020 a group at the Van Andel Institute and Harvard published a second, CRISPR-made osteocalcin knockout (Diegel et al., PLoS Genetics, PMID 32463812). Its mice had no detectable osteocalcin, and showed changes in bone mineral quality — but serum glucose and male fertility did not differ significantly from normal littermates, and bone mass and strength were unchanged. The authors wrote: "We cannot explain the absence of endocrine effects", and donated the strain so others could test it.
| What is claimed | How firm |
|---|---|
| Binds calcium and bone mineral once carboxylated | Human bone biochemistry (UniProt) |
| Blood level tracks bone formation | Used clinically; outcome in over a thousand trials |
| Regulates blood sugar as a hormone | Mouse genetics; contradicted by a second knockout strain (2020) |
| Raises testosterone; acts on the brain | Mouse work; no human treatment trial found |
No human trial has given osteocalcin as a treatment, and nothing in this record shows that raising it does anything in people.
Where it stands
- Not a medicine; a laboratory measurement and a bone-matrix protein.
- Research volume: 20,738 PubMed records name it in the title or abstract, 1,169 indexed as randomised trials; ClinicalTrials.gov lists 678 studies mentioning it (searched 2026-11-02).
Related reading
Adiponectin, the fat-tissue hormone the 2007 paper found osteocalcin could induce in fat cells, and parathyroid hormone, which sets the calcium balance bone answers to. PTHrP is the other bone-related protein measured as a blood test.